If a count noun refers to one unspecific item (not a whole category), use the indefinite article a (or an). A and an usually mean “one among many” but can also mean “any one.”
The noun dictionary refers to “one unspecific dictionary” or “any dictionary.”
The noun apartment refers to “any apartment close to the lake,” not a specific apartment.
Choosing articles for common nouns
Exercises:
Using articles 1
Using articles 2
Using articles 3
Articles and types of nouns